1. FIELD OF INVENTION
This invention pertains to a detector used for sensing cracks in an elongated web, and in particular, to a detector for detecting the size and shape of edge cracks in a paper web in a papermaking process.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Most paper today is made in a continuous sheet on large papermaking machines from a web which undergoes several processing steps. The web is usually 20 feet wide or greater and is made in a continuous manner at very high speeds of, for example, 3000 ft/min. Because the profitability of papermaking machines is directly related to their continuous operation, interruption of full production is extremely costly and, therefore, factors which cause such interruptions are studied intensely to reduce such incidents to a minimum. One factor which results in the interruption of the papermaking machine is a crosswise tear in the paper web. Whenever such a tear occurs the papermaking machine must be shut down totally or partly until the problem is resolved. One cause of these tears are cracks formed at the web edges as the web travels through the machine. While often minor cracks can be tolerated with no problems, larger cracks can propagate across the sheet to form a tear.
Therefore, it is important to monitor cracks on the web edges and to analyze their formation and behavior in a papermaking machine to control web tearing. A better understanding of crack formation and propagation would also lead to improved preventive maintenance and emergency procedures. In fact, numerous devices are known for detecting cracks in moving elongated webs for such purposes. For example, devices are known which scan the web material in the direction of its width to find cracks as taught in U.S. Pat. No. 4,160,913 to Brenholdt, U.S. Pat. No. 4,247,204 to Merlen et al., U.S. Pat. No. 4,335,316 to Glanz et al. and U.S. Pat. No. 4,791,304 to Iida. Other devices are also known which detect tears and holes as well as web edges using stationary light transmission/detection systems. In these systems, light is transmitted toward the web material and is detected by detectors on the other side of the web material when the light is allowed to pass through the web material by cracks or holes therein. For example, such devices are taught in U.S. Pat. No. 2,735,329 to Meunier (sheet metal), U.S. Pat. No. 4,559,451 to Curl, U.S. Pat. No. 4,652,124 to Bowen et al. U.S. Pat. No. 4,680,806 to Bolza Schunemann, U.S. Pat. No. 4,709,157 to Shimizu et al., U.S. Pat. No. 4,728,800 to Surka and U.S. Pat. No. 4,788,442 to Sabater et al.
The above-mentioned devices detect tears in fabric webs which are relatively thick and move at relatively slow running speeds. Until now, because of the high speed of papermaking machines and the resultant flutter of the edges of the light weight paper sheets, it has been very difficult to sense and monitor edge cracks properly. Moreover, such systems do not determine the size and shape or direction of cracks in such a manner that tears in the web can be more accurately predicted. The present invention is designed to improve performance by determining the size and the shape of cracks in the web even in the presence of sheet flutter.